Generally, a hard disk drive functions as an auxiliary memory unit of a computer, which is comprised of a platter, a head, a spindle motor, a head arm and a printed circuit board. The hard disk drive helps to operate a system of the computer either by reading out and regenerating information stored at the platter through the head or by writing new information on the platter through the head.
In the construction of the above-mentioned hard disk drive, the platter is a metallic circular plate coated with magnetic material, functioning to write various data. The platter is rotated with a plurality of rows stacked on a rotatable shaft. This rotatable shaft is named a spindle shaft. A motor for rotating the spindle shaft is named a spindle motor.
The head for reading/writing data stored at the platter is connected to the head arm so as to access desired information addresses. This head arm is driven by a head actuator, which is named a voice coil motor (VCM). A conventional spindle motor will be described below.
FIG. 1 is an exploded perspective view of a conventional spindle motor for hard disk drives employing at least one ball bearing, and FIG. 2 is a cross sectional view of the spindle motor of FIG. 1.
As shown in FIGS. 1 and 2, the conventional spindle motor 10 for hard disk drives employing at least one ball bearing comprises a base 11 formed as a lower portion of the spindle motor, a spindle shaft 12 fitted at a center of the base in a vertical direction, a first ball bearing 13 fitted on a lower portion of the spindle shaft 12 positioned on the upper side of the base 11, a stator 14 fitted around the first ball bearing 13 and constructed for a coil 14b to be wound around a core 14a, a second ball bearing 15 fitted on an upper portion of the spindle shaft 12, a hub 16 supported to be rotatable about the fist and second ball bearings 13 and 15 and constructed to cover the upper portion of the base 11, and an annular permanent magnet 17 fitted on an inner circumferential surface of a lower portion of the hub 16 and generating driving force for rotating the hub 16 through the magnetic field produced in cooperation with the coil 14b. 
In the conventional spindle motor 10 for hard disk drives employing at least one ball bearing constructed as mentioned above, when power is supplied to the coil 14b of the stator 14, a magnetic field (not shown) is established between the coil 14b and the permanent magnet 17. The magnetic field between the coil 14b and the permanent magnet 17 allows the hub 16 to be rotated in one direction.
However, construction of rotating the hub 16 by use of ball bearings 13 and 15 makes it impossible to implement high-speed driving with a strict rotational precision, which results in generating noise and vibration when the ball bearing rotates at a high speed. The following description will be made regarding the construction of an aerodynamic bearing shown in FIG. 3.
FIG. 3 is a cross-sectional view of a conventional spindle motor for hard disk drives employing at least one aerodynamic bearing.
The conventional spindle motor 20 for hard disk drives employing at least one aerodynamic bearing shown in FIG. 3 includes a base 21 formed as a lower portion of the spindle motor, a first bearing 22 fined on an upper central portion of the base 21, a stator 23 fitted around the first ball bearing 22 and constructed to enable a coil 23b to be wound around a core 23a, a spindle shaft 24 fitted on an upper central portion in a vertical direction, a second bearing 25 fitted on an upper portion of the spindle shaft 24, a supported hub 26 that is rotatable about the spindle shaft 24 and constructed to cover the upper portion of the base 21, first and second aerodynamic bearings 27 and 28 fitted on an inner upper portion of the hub 26 and generating aerodynamic pressure for smoothly rotating the hub 26 about the spindle shaft 24, and a permanent magnet 27 fitted on an inner circumferential surface of a lower portion of the hub 16 and generating driving force for rotating the hub 26 through the magnetic field produced in cooperation with the coil 23b. 
In the conventional spindle motor 20 for hard disk drives employing at least one aerodynamic bearing constructed as mentioned above, when power is supplied to the coil 23b of the stator 23, a magnetic field (not shown) is established between the coil 23b and the permanent magnet 27. The magnetic field between the coil 23b and the permanent magnet 27 allows the hub 26 to be rotated in one direction.
Once the hub 26 rotates, air begins to flow on the inner surfaces of the first and second aerodynamic bearings 27 and 28. The faster the hub 26 rotates, the stronger the air flows. As a result, the flow of the air is changed into a layer of air having a predetermined rigidity between the first and second aerodynamic bearings 27 and 28, the spindle shaft 24, the first bearing 25, and the second bearing 22 in proportion to the rotational speed of the hub 26. Therefore, the hub 26 with a mounted platter (not shown) rotates about the spindle shaft 24 while overcoming the imposed load and disturbance from the air layer between the spindle shaft 24 and the hub 26 acting as a bearing.
However, the conventional spindle motor for hard disk drives employing at least one aerodynamic bearing, constructed as mentioned above, enables rigidity of the air layer to be improved at a low-speed rotation, but rigidity of the air layer is maintained almost constantly without an increase in proportion to the rotational speed when the motor rotates over a fixed speed.
Further, the conventional spindle motor for hard disk drives employing at least one aerodynamic bearing constructed, as mentioned above, is designed so that the base is assembled with the first bearing but the spindle shaft is assembled with the second bearing, so that the assembled two sets maintain a predetermined size of air gap with respect to the aerodynamic bearings. Therefore, there are problems in that the spindle motor has a reduced assembly capability and has a difficulty in constantly maintaining a constant thickness of the air gap. Moreover, the spindle motor is designed so that the hub is supported around the spindle shaft via the air gap without putting the hub into direct contact with the spindle shaft Therefore, during an initial starting, the spindle motor is subjected to malfunction, attrition losses of the aerodynamic bearings as well as the first and second bearings, noise and vibration, all of which are caused by friction.